1. Field of the Invention
The invention relates generally to an electric power steering apparatus, and more specifically to an electric power steering apparatus configured to provide steering assistance by using an axial driving force that is applied to a rack shaft when the rotary torque generated by an electric motor is transmitted through a toothed belt to a ball screw nut.
2. Description of Related Art
This type of electric power steering apparatus is described in, for example, Japanese Patent Application Publication No. 2013-184682. In the electric power steering apparatus described in JP 2013-184682 A, in the case where the amount of axial travel (the stroke amount) of a rack shaft is limited when any one of rack ends provided at respective ends of the rack shaft and a housing come into contact with each other in the axial direction, while the rotation of a driven toothed-pulley is restricted at the stroke end, the tension of a part of a toothed belt is increased by the rotary inertia of a driving-side member, such as an electric motor, causing a possibility that the toothed belt will jump the teeth of the pulley (the toothed belt will fail to mesh properly with the teeth of the pulley). The phenomenon in which the toothed belt jumps the teeth of the pulley occurs, for example, because an increase in the tension of a part of the toothed belt causes a ball screw nut to tilt relative to a ball screw shaft such that the distance between the center of rotation of a driving toothed-pulley and the center of rotation of the driven toothed-pulley decreases. In this case, as the distance between the centers of rotation of these pulleys decreases, the support stiffness with which the driving toothed-pulley and the driven toothed-pulley support the toothed belt decreases, so that the toothed belt is more likely to jump the teeth of the pulley.